As mentioned in this article, it appears that the creators of the new Raspberry Pi 4 did not use the correct specs for building the usb-c connections.
https://arstechnica.com/gadgets/2019/07/raspberry-pi-4-uses-incorrect-usb-c-design-wont-work-with-some-chargers/"
This means that some chargers may not work correctly with the new single board computer.
Luckily, the power supply I purchased with my new pi 4 was the Raspberry Pi 4 model, so it works as expected.
Definitely worth a read if you have or are considering picking up one of these units.